This project is designed to study morphological and functional changes in the neural substrate for a learned behavior. Zebra finches (Poephila guttata) will be used as an animal model in which to investigate factors regulating structural plasticity in developing and adult brain, trophic interactions in vertebrate fore brain, neural and hormonal mechanisms underlying the development of a complex learned behavior, and sex differences in neural morphology and cellular processes. 1. Zebra finches learn the sounds used for communication during a restricted period of development, and song learning and behavior are controlled by well-defined neural circuits. The contribution of proprioceptive feedback from vocal muscles to song learning has never been evaluated, although sensory axons from the vocal organ are known to terminate throughout the trigeminal tract. We have recently shown that lesions of the telencephalic end- station of the trigeminal system disrupt vocal behavior in juvenile birds during song learning, but have no effect on maintenance of song patterns by adult birds. We will map out the precise axonal connections and topographic relationships of this telencephalic nucleus in order to determine if afferent fibers from the vocal organ project to this area, and if so whether they terminate selectively in a region that is necessary for normal vocal development. 2. We have previously demonstrated that the major thalamic relay of the song-control system initially makes an exuberant projection to its cortical target nucleus; subsequently both the target nucleus and its thalamic input regress. We will label individual thalamic axon terminals at different stages of development in order to determine whether single terminal arbors are expanding and regressing, as has recently been demonstrated in the visual system, and whether the timing of these events correlates with naturally-occurring cell death or the extent to which this circuit contributes to vocal behavior. We will also lesion this thalamic nucleus in order to examine induced cell death - i.e., we will ask whether afferent input is necessary for neuronal survival in the target nucleus in both developing and adult animals. 3. We have discovered that cell division is much higher in the germinal epithelium of the telencephalon in males than in females during late stages of post-hatch development. This is the first demonstration of a robust sex difference in neurogenesis in vertebrate brain, and suggests that sex dimorphisms in neural circuits underlying vocal behavior may emerge via regulation of neuronal proliferation. We will begin to explore this remarkable result by investigating the role of sex hormones on cell division in the telencephalon.